Control of a hydrostatic gearbox

ABSTRACT

The invention relates to control of a hydrostatic gearbox, comprising a hydraulic pump ( 2 ), provided for pumping in a first pump-side main line ( 5   a ) or a second pump-side main line ( 6   a ). The hydrostatic gearbox further comprises a hydraulic motor ( 3 ), connected to a first motor-side main line ( 5   b ) and a second motor-side main line ( 6   b ). The first pump-side main line ( 5   a ) and the second pump-side main line ( 6   a ) may be connected to the first motor-side main line ( 5   b ) or the second motor-side main line ( 6   b ) by means of a brake valve unit ( 19 ). The first motor-side main line ( 5   b ) or the second motor-side main line ( 6   b ), arranged downstream of the hydraulic motor ( 3 ), may be connected to a tank volume ( 12 ) in a throttled manner, depending on the pressure therein.

The invention relates to a control system for a hydrostatic transmissionin an open circuit.

In an open circuit, a hydraulic pump draws in pressure medium from atank volume and delivers it under pressure to a hydraulic motor via amain line. The hydraulic motor here serves, for example, to drive avehicle, with the pressure medium which flows through the hydraulicmotor flowing through a further main line, connected downstream to thehydraulic motor, back into the tank volume. If such a system is inoverrun condition, the hydraulic motor starts to draw in pressure mediumfrom the main line pressurised by the hydraulic pump and now acts, inturn, as a pump which delivers the pressure medium towards the tankvolume.

In order to prevent, in such an arrangement, for example a vehicledriven by the hydrostatic transmission from getting into an unbrakeddriving condition, it is known for example from DE 41 29 667 A1 to use abrake valve through which the return flow from the hydraulic motoracting as a pump takes place in a throttled manner. During normaldriving, the brake valve is brought into a switching position by thedelivery pressure of the hydraulic pump counter to a spring force, inwhich position the pressure medium can return through the brake valve inan unthrottled manner. On changing to the overrun condition, thedelivery pressure of the hydraulic pump falls considerably, so that thebrake valve returns to its starting position. In this starting position,the main line arranged downstream of the hydraulic motor is connected tothe tank volume via a throttling point.

The hydraulic motor, acting as a pump in the overrun condition, buildsup a pressure in its main line situated downstream on account of thisthrottling point, thereby resulting in the intended braking action. Thereturn of the brake valve to its neutral position, in which thedownstream main line functioning as a return line is connected to thetank volume via a throttling point, takes place solely on account of twocompression springs which bring the piston of the brake valve into acentral position.

In order, in the event of a great pressure increase in the downstreammain line, to prevent the pressure in the main line from exceeding acritical pressure, two pressure limiting valves are provided, via whichthe two main lines are short-circuited by the pressure when a thresholdvalue is exceeded.

The system described has the disadvantage that the brake valve isbrought into its central position by the force of the restoring springsand no control takes place during the braking process. In this centralposition, a certain throttling cross-section is fixed, and produces thebraking action. The actuation of the brake valve takes placeindependently of the pressure conditions prevailing in the main lines onthe side of the hydraulic motor, so that control with respect to theload of the hydraulic motor cannot take place.

A further disadvantage is that, in order to limit the pressure in thedownstream main line, short-circuiting of the two main lines is carriedout. Some of the pressure medium circulating in this case does nottherefore flow through the tank volume and any additionally providedfilters and coolers.

The object of the invention is to provide a control system for ahydrostatic transmission in an open circuit, in which a braking actionis brought about in dependence on the pressure produced by the hydraulicmotor in its downstream main line.

The object is achieved by the control system according to the inventionhaving the features of claim 1.

According to the invention, a main line arranged downstream of thehydraulic motor is connected to a tank volume in the overrun conditionby a brake valve unit. The connection takes place in a throttled manner,the throttling being dependent on how high the pressure is in the mainline arranged downstream of the hydraulic motor. At a high pressure,i.e. with a strong pumping action of the hydraulic motor, only slightthrottling takes place. Such slight throttling is accordingly feltmerely as a small braking action, so that the sharp braking jolt whichoccurs with constant throttling is eliminated.

The subclaims relate to advantageous developments of the hydrostatictransmission according to the invention.

In this regard, it is particularly advantageous to design the brakevalve unit such that, in addition to being subjected to the pressureprevailing in the motor-side main line arranged downstream of thehydraulic motor, it is also subjected to the delivery pressure of thehydraulic pump.

As a result, during normal driving, the motor-side main line arrangeddownstream of the hydraulic motor is likewise connected to the tankvolume. By an appropriate choice of the measuring surfaces which aresubjected to pressure, it is possible here, for normal driving, toenable a virtually unthrottled return of the pressure medium towards thetank volume.

Furthermore, it is particularly advantageous to provide a brake valveunit which has a rest position in which a flow of pressure medium comingfrom the hydraulic motor towards the tank volume is not possible.Through such a complete interruption of the possibility of flowing back,a vehicle standing on a slope, for example, is prevented from runningoff independently.

For a hydrostatic drive whose travelling direction valve has an idleposition, it is furthermore particularly advantageous for both the mainline upstream of the hydraulic motor and the main line downstream of thehydraulic motor to be connected to the tank volume in the idle position.

Preferred exemplary embodiments of the control system, according to theinvention, for a hydrostatic transmission are illustrated in the drawingand explained in more detail by means of the following description. Inthe drawing:

FIG. 1 shows a first exemplary embodiment of a circuit diagram of acontrol system according to the invention,

FIG. 2 shows a second exemplary embodiment of a circuit diagram of acontrol system according to the invention,

FIG. 2 a shows a modified brake valve,

FIG. 3 shows a third exemplary embodiment of a circuit diagram of acontrol system according to the invention,

FIG. 4 shows a fourth exemplary embodiment of a circuit diagram of acontrol system according to the invention,

FIG. 5 shows a fifth exemplary embodiment of a circuit diagram of acontrol system, according to the invention, of a hydrostatictransmission,

FIG. 5 a shows a modified brake valve of the exemplary embodiment ofFIG. 5, and

FIG. 6 shows a sixth exemplary embodiment of a circuit diagram of acontrol system according to the invention.

FIG. 1 shows a first exemplary embodiment of a circuit diagram of acontrol system, according to the invention, of a hydrostatictransmission 1. The hydrostatic transmission 1 comprises a hydraulicpump 2, which in the exemplary embodiment illustrated can be operatedwith an adjustable delivery volume. The pressure medium delivered by thehydraulic pump 2 drives a hydraulic motor 3, the absorbing volume ofwhich is likewise adjustable.

To set the direction of rotation of the hydraulic motor 3, the hydraulicpump 2, which is designed for delivery in one direction only, isconnected via a travelling direction valve 4 to a first pump-side mainline 5 a or a second pump-side main line 6 a. Depending on theparticular driving situation, the first pump-side main line 5 a isconnected to the first motor-side main line 5 b. If the first pump-sidemain line 5 a is connected via the travelling direction valve 4 to thehydraulic pump 2, as will be explained in detail later, the firstpump-side main line 5 a and, on top of that, also the first motor-sidemain line 5 b are pressurised by the hydraulic pump 2. The pressuremedium consequently drives the hydraulic motor 3 and flows, downstreamof the hydraulic motor 3, back towards a tank volume 12 via the secondmotor-side main line 6 b and the second pump-side main line 6 a.

To adjust the variable absorbing volume of the hydraulic motor 3, thereis provided an adjusting device 7 consisting essentially of a settingvalve 8 and a setting unit 9. The setting unit 9 comprises a cylinder,in which there is arranged a setting piston 10 which divides thecylinder into a first pressure chamber 11 a and a second pressurechamber 11 b. Arranged between the first pump-side main line 5 a and thesecond pump-side main line 6 a is a shuttle valve 13, by means of whichin each case the higher of the pressures prevailing in the pump-sidemain lines 5 a, 6 a is present in a setting pressure supply line 14. Thesetting pressure supply line 14 is connected to the second pressurechamber 11 b. In addition, the setting pressure supply line 14 isconnected to an inlet of the setting valve 8. When the pressure in thesetting pressure supply line 14 increases, the inlet of the settingvalve 8 is connected via a throttle 15 to the first pressure chamber 11a. If the pressures in the first pressure chamber 11 a and the secondpressure chamber 11 b are equal, a resultant force acts on the settingpiston 10, since the piston area in the first pressure chamber 11 a isgreater than in the second pressure chamber 11 b.

By contrast, if the pressure in the setting pressure supply line 14falls, the setting valve 8 is displaced in the opposite direction by theforce of a compression spring which acts in the opposite direction tothe pressure in the setting pressure supply line 14, the first pressurechamber 11 a being increasingly connected to the tank volume 12. Duringnormal operation, this means that, for example during the driving-offprocess, during which the pressure prevailing in the first pump-sidemain line 5 a is greatly increased, the hydraulic motor 3 is displacedtowards maximum absorbing volume and thus maximum torque. If thepressure in the first pump-side main line 5 a is reduced by increasingspeed of the vehicle after the driving-off process, the pressure in thesetting pressure supply line 14 thus also falls. This decreasingpressure in the setting pressure supply line 14 brings about a movementof the setting valve 8 and thus relief of the first pressure chamber 11a to the tank volume 12, so that the hydraulic motor 3 is pivotedtowards a smaller absorbing volume until a state of equilibrium isestablished.

The hydraulic pump 2 delivers the pressure medium to a pressure line 16,the pressure line 16 being connectable via the travelling directionvalve 4 to the first pump-side main line 5 a or the second pump-sidemain line 6 a. To this end, starting from the rest position illustratedin FIG. 1, the travelling direction valve 4 can be switched to a firstor second switching position 20 or 21, respectively. The rest positionof the travelling direction valve 4 is governed by a first compressionspring 22 and a second compression spring 23, which keep the travellingdirection valve 4 in a central position. In order to connect thepressure line 16 to the corresponding pump-side main line 5 a or 6 a inaccordance with the desired delivery direction, a first switching magnet24 and a second switching magnet 25 are provided, respectively. For thesubsequent explanations regarding the functioning of a brake valve unit19 is discussed below, it is assumed that the travelling direction valve4 is in its first switching position 20, in which the pressure line 16,into which the hydraulic pump 2 pumps the pressure medium drawn in fromthe tank volume 12 via a suction line 17, is connected to the firstpump-side main line 5 a.

Simultaneously, the second pump-side main line 6 a is connected to atank line 18 by the travelling direction valve 4 in the first switchingposition 20, which tank line connects the second pump-side main line 6 ato the tank volume 12 via a spring-loaded check valve 26.

In the first exemplary embodiment illustrated in FIG. 1, the firstpump-side main line 5 a is connected to the first motor-side main line 5b by the brake valve unit 19 via a first check valve 27. The first checkvalve 27 is arranged such that it opens towards the hydraulic motor 3.Furthermore, a second check valve 28 is provided, which likewise openstowards the hydraulic motor 3 and which is therefore in its closedposition for the delivery direction described, so that it is notpossible for the pressure medium to flow back directly into the secondpump-side main line 6 a via the second motor-side main line 6 b.

A possibility for the delivered pressure medium to flow back from thesecond motor-side main line 6 b towards the tank volume 12 is providedvia a brake valve 29. To this end, the brake valve 29 in a first endposition 30 connects a first branch line 31 of the second motor-sidemain line 6 b to a return connecting line 32. The return connecting line32 is connected to the first pump-side main line 5 a via a first returncheck valve 33 and to the second pump-side main line 6 a via a secondreturn check valve 34. The first return check valve 33 and the secondreturn check valve 34 are each arranged such that they open towards thefirst pump-side main line 5 a and the second pump-side main line 6 a,respectively.

A first measuring surface 35 of the brake valve 29 is connected to thesecond motor-side main line 6 b via a first brake pressure line 36. Ahydraulic force, governed by the pressure prevailing in the secondmotor-side main line 6 b, thus acts on the first measuring surface 35.The first measuring surface 35 is oriented such that the hydraulic forceacting there deflects the brake valve 29 from its rest position towardsthe first end position 30 counter to the force of a first centringspring 37.

In addition, a second measuring surface 38, connected by a firstpressure take-off line 39 to the first pump-side main line 5 a, isformed on the brake valve 29. The first measuring surface 35 and thesecond measuring surface 38 are formed in the same direction on thebrake valve 29, so that both the hydraulic force acting on the firstmeasuring surface 35 and the hydraulic force acting on the larger secondmeasuring surface 38 deflect the brake valve 29 towards its first endposition 30. The transition between the rest position of the brake valve29 and the first end position 30 is continuous here, so that the brakevalve 29 forms an adjustable throttle in dependence on the pressurepresent at the first measuring surface 35 and the second measuringsurface 38, respectively.

In the rest position of the brake valve 29, the connection between thefirst branch line 31 of the second motor-side main line 6 b and thereturn connecting line 32 is completely broken, while in the first endposition 30 of the brake valve 29, the connection from the first branchline 31 to the return connecting line 32 is virtually unthrottled.

In the described first switching position 20 of the travelling directionvalve 4, during a normal driving situation in which a vehicle isaccelerated or driven on the level via the hydrostatic transmission 1,the first pump-side main line 5 a and the first motor-side main line 5 bare pressurised and the hydraulic motor 3 is driven. By contrast, thesecond motor-side main line 6 b, arranged downstream of the hydraulicmotor 3, is relieved towards the tank volume 12 via the brake valve 29.To this end, the brake valve 29 is brought into its first end position30, by the delivery pressure of the hydraulic pump 2 acting on thesecond measuring surface 38 via the first pressure take-off line 39, andthus connects the first branch line 31 to the return connecting line 32.Consequently, the second return check valve 34 opens and thus frees theflow path for the flowing-back pressure medium via the second pump-sidemain line 6 a and the travelling direction valve 4, the tank line 18 andthe spring-loaded check valve 26. The check valve 26 ensures that thereis a low residual pressure in the line system here.

If a driving situation now occurs, for example when travelling downhillor braking, in which the vehicle is not driven by the hydraulic motor 3but conversely the vehicle operates the hydraulic motor 3 in the senseof a pump, the pressure in the first pump-side main line 5 a falls. Withthe pressure falling in the first pump-side main line 5 a, the hydraulicforce acting on the brake valve 29 at the second measuring surface 38also falls simultaneously, so that the brake valve 29 is moved towardsits rest position by the force of the first centring spring 37. Throughthe displacement of the brake valve 29 towards its rest position, theconnection of the first branch line 31 to the return connecting line 32is increasingly throttled. This increasing throttling offers anever-increasing flow resistance to the flowing-back pressure medium,which consequently leads to an increase of the pressure in the secondmotor-side main line 6 b, situated downstream of the hydraulic motor 3.

This pressure, which increases with increasing throttling, in the secondmotor-side main line 6 b travels, via the first brake pressure line 36,to the first measuring surface 35 and acts there, once again, counter tothe first centring spring 37. Since the area of the first measuringsurface 35 is smaller than the area of the second measuring surface 38,only a throttled connection is opened between the first branch line 31and the return connecting line 32.

As a result of this throttled connection, the hydraulic motor 3 operatedas a pump has to perform work to convey the pressure medium towards thetank volume 12, thereby achieving the desired braking action.

Since the throttling carried out in the brake valve 29 is dependent onthe level of the pressure present at the first measuring surface 35, notonly is an improvement in comfort during braking achieved, but also theuse of additional pressure limiting valves is unnecessary. If need be,they may be used additionally for safety reasons. This function islikewise already performed by the brake valve 29, since with risingpressure in the second motor-side main line 6 b, situated downstream ofthe hydraulic motor 3, a greater through-flow cross-section through thebrake valve 29 is freed.

Moreover, appropriate dimensioning of the spring rate of the firstcentring spring 37 and the size of the first measuring surface 35 makesit possible for the connection between the first branch line 31 and thereturn connecting line 32 to be completely broken, as long as a certainthreshold value for the pressure in the second motor-side main line 6 bis not exceeded. This makes it possible, for example, to park thevehicle on a slope, so that it cannot start moving by itself, incontrast to a fixed throttle, since the hydraulic motor 3 working as apump is blocked on account of the line interruption.

The above explanations regarding a flow direction from the firstpump-side main line 5 a via the first motor-side main line 5 b throughthe hydraulic motor 3 and back via the second motor-side main line 6 band the second pump-side main line 6 a to the tank volume 12 applyanalogously also to a reverse delivery direction, as occurs on reversalof the travelling direction. The travelling direction valve 4 is broughthere into its second switching position 21 by the second switchingmagnet 25. In this case, during normal driving, the pressure mediumflows towards the hydraulic motor 3 via the second check valve 28, athird measuring surface 38′ of the brake valve 29 being subjected to ahydraulic force counter to a second centring spring 37′ via the secondpressure take-off line 39′. Through the resultant deflection of thebrake valve 29, a second branch line 31′ of the first motor-side mainline 5 b is connected to the return connecting line 32 if the brakevalve 29 is in its second end position 32. The connection is virtuallyunthrottled.

If the vehicle gets into overrun condition, in the delivery directiondescribed last, a fourth measuring surface 35′ of the brake valve 29,which is smaller than the third measuring surface 38′, is subjected tothe correspondingly increased pressure in the first motor-side main line5 b, now situated downstream of the hydraulic pump 3, so that the brakevalve 29 once again frees a throttled cross-section towards the tankvolume 12, via which the pressure medium returns from the firstmotor-side main line 5 b via the second branch line 31′. To subject thefourth measuring surface 35′ to a pressure, the first motor-side mainline 5 b is connected to the fourth measuring surface 35′ via a secondbrake pressure line 36′.

FIG. 2 illustrates a further exemplary embodiment with an alternativedesign of a brake valve unit 19′. The construction of the hydrostatictransmission 1 corresponds essentially to the construction of thehydrostatic transmission illustrated in FIG. 1, so that identicalelements are provided with the same reference symbols. In contrast tothe embodiment explained in FIG. 1, however, the brake valve 29 is notdirectly subjected to a pressure at its first measuring surface 35 andits fourth measuring surface 35′ from the second motor-side main line 6b and the first motor-side main line 5 b, respectively. To control thepressure prevailing at the first measuring surface 35 and the fourthmeasuring surface 35′, use is made here of a pilot control valve 45having a first outlet 46, which is connected to the first measuringsurface 35 via a first brake pressure line section 47. A second outlet46′ of the pilot control valve 45 is connected to the fourth measuringsurface 35′ via a second brake pressure line section 47′.

The pilot control valve 45 is kept by two restoring springs 48, 48′ inits rest position, in which the first and second outlet 46 and 46′ areseparated from an inlet 49 of the pilot control valve 45. The inlet 49of the pilot control valve 45 is connected to the first motor-side mainline 5 b and the second motor-side main line 6 b via a shuttle valve 50,so that in each case the higher of the pressures of the first motor-sidemain line 5 b and the second motor-side main line 6 b prevails at theinlet 49.

If the travelling direction valve 4 is in its first switching position20, the driven vehicle being in overrun condition, then, as has alreadybeen explained with reference to FIG. 1, the first check valve 27 isopen on account of the delivery pressure of the hydraulic pump 2,whereas the second check valve 28 is closed. Owing to the overruncondition and the hydraulic motor 3 therefore acting as a pump, thepressure in the second motor-side main line 6 b rises and the shuttlevalve 50 is in its position illustrated in FIG. 2. The increasedpressure of the second motor-side main line 6 b therefore acts on theinlet 49 of the pilot control valve 45.

Simultaneously, via a first brake pressure measuring line section 51, afirst brake pressure measuring surface 52 of the pilot control valve 45is subjected to a hydraulic force corresponding to the pressureprevailing in the second motor-side main line 6 b, so that the pilotcontrol valve 45 is deflected from its rest position towards a firstcontrol position 53. In dependence on the hydraulic force at the firstbrake pressure measuring surface 52 and the opposite force of the firstrestoring spring 48, the pilot control valve 45 continuously opens athrough-flow connection from the inlet 49 to the first outlet 46. Whenthe pilot control valve 45 reaches its first control position 53, theconnection is fully open, so that the pressure of the second motor-sidemain line 6 b is present at the first measuring surface 35 of the brakevalve 29.

The pilot control valve 45 is, once again, symmetrically constructed, sothat it functions analogously in the reverse flow direction. To thisend, a second brake pressure measuring surface 52′, which is connectedto the first motor-side main line 5 b via a second brake pressuremeasuring line section 51′, is formed on the pilot control valve 45. If,in the opposite flow direction, the pressure in the first motor-sidemain line 5 b exceeds the pressure in the second motor-side main line 6b, the inlet 49 of the pilot control valve 45 is connected to the firstmotor-side main line 5 b via the shuttle valve 50.

The functioning and construction of the brake valve 29 is identical tothe functioning and construction of the brake valve 29 from FIG. 1.However, by using the pilot control valve 45, it is possible toinfluence the brake pressure present at the measuring surfaces 35 and35′. In particular, the time characteristic can advantageously beadapted to the particular use conditions of the vehicle and to thevehicle itself.

Instead of the brake valve 29 as described in the exemplary embodimentsof FIG. 1 and FIG. 2, a modified brake valve 129 may also be used withparticular advantage in both exemplary embodiments. The modified brakevalve 129 is illustrated in FIG. 2 a. If the modified brake valve 129 isin its rest position, the first branch line 31 is connected in athrottled manner to the second branch line 31′. Through the throttledconnection, the control stability of the system is improved.

A further embodiment of the control system according to the invention isillustrated in the hydraulic circuit diagram of FIG. 3. A brake valveunit 60 provided therein consists essentially of a first brake valve 61and a second brake valve 61′. The following explanations which refermerely to the first brake valve 61 apply analogously also to the secondbrake valve 61′, reference symbols which correspond to each other beingused as apostrophised reference symbols in connection with the secondbrake valve 61′.

The first brake valve 61 has a first connection 62 and a secondconnection 63, which have no through-flow connection in the restposition of the first brake valve 61. The first brake valve 61 is keptin the rest position by a spring 64 as long as there is no pressure,deflecting the brake valve 61 from its rest position towards an endposition 67 counter to the force of the spring 64, present at its firstmeasuring surface 65 or its larger second measuring surface 66. Thefirst pump-side main line 5 a is connected to the first motor-side mainline 5 b via a first check valve 27, arranged in a bypass line 68. Ifthe travelling direction valve 4 is in its first switching position 20as already explained, the first pump-side main line 5 a is pressurisedby the hydraulic pump 2, the pressure travelling to the first motor-sidemain line 5 b via the bypass line 68 and the first check valve 27, whichopens towards the hydraulic motor 3.

The pressure prevailing in the first pump-side main line 5 a acts on thesecond measuring surface 66′ of the second brake valve 61′, for whichpurpose the second measuring surface 66′ of the second brake valve 61′is connected to the first motor-side main line 5 a via a third pressuretake-off line 39′. The delivery pressure of the hydraulic pump 2 actingon the second measuring surface 66′ of the second brake valve 61′deflects the valve from its rest position towards its end position 67′counter to the force of the spring 64′.

In the end position 67′ of the second brake valve 61′, the secondmotor-side main line 6 b is connected to the second pump-side main line6 a and, despite the closed second check valve 28 of the bypass line68′, it is still possible for the pressure medium delivered by thehydraulic motor 3 to flow back towards the tank volume 12.

If, on account of a braking process, the hydrostatic transmission 1 getsinto overrun condition, where the hydraulic motor 3 acts as a pump, thepressure in the first pump-side main line 5 a falls. Accordingly,greater throttling takes place through the second brake valve 61′, whichis displaced towards its rest position by the spring 64′ counter to thedecreasing hydraulic force acting on the second measuring surface 66′.The greater throttling simultaneously brings about a pressure rise inthe second motor-side main line 6 b. The increased pressure in thesecond motor-side main line 6 b is transmitted to the first measuringsurface 65′ of the second brake valve 61′ via a brake line section 70′.To this end, the brake line section 70′ is connected via a connectingline 71 to a take-off line 73′, in which check valve 72′ opening towardsthe second brake valve 61′ is arranged. The simultaneous change of thepressures present at the measuring surfaces is used particularlyadvantageously to enable gentle initiation of the braking process. Tothis end, the ratios of the surfaces and the spring rate of theoppositely acting spring are matched to one another in all the brakevalves used.

The hydraulic force thus acting on the first measuring surface 65′ ofthe second brake valve 61′ deflects the brake valve 61′ from its restposition towards the end position 67′, so that a throttled connection isestablished between the second motor-side main line 6 b and the secondpump-side main line 6 a. The pumping hydraulic motor 3 performs work atthe throttling point, the intensity of the throttling being dependent onthe pressure prevailing in the second motor-side main line 6 b. Anexcessive rise of the pressure in the second motor-side main line 6 b isprevented, since a pressure increase also brings about an increase ofthe hydraulic force at the first measuring surface 65′ of the brakevalve 61′ and consequently the flow cross-section is enlarged.

FIG. 4 illustrates a similar exemplary embodiment in which a first brakevalve 61 and a second brake valve 61′ are provided. In contrast to theexemplary embodiment of FIG. 3, however, the first measuring surfaces 65and 65′ are in this case not connected to the first and secondmotor-side main line 5 b and 6 b, respectively, via check valves.Instead, the first measuring surface 65 is directly connected to thefirst motor-side main line 5 b via a connecting line 75 and the firstmeasuring surface 65′ of the second brake valve 61′ is directlyconnected to the second motor-side main line 6 b via a second connectingline 75′.

A hydraulic circuit diagram for a fifth exemplary embodiment of thecontrol system according to the invention is illustrated in FIG. 5. Inthis embodiment, the brake valve unit 80 comprises a brake valve 81. Thebrake valve 81 has a first connection 82, to which the first pump-sidemain line 5 a is connected. A second connection 83 of the brake valve 81is connected to the first motor-side main line 5 b. Correspondingly, athird connection 84 and a fourth connection 85 are connected to thesecond pump-side main line 6 a and the second motor-side main line 6 b,respectively. If the brake valve 81 is in its central position 86, theconnections 82 to 85 have no connection through the brake valve 81.

If the first pump-side main line 5 a is pressurised by the hydraulicpump 2 and the travelling direction valve 4, the delivery pressure ofthe hydraulic pump 2 is transmitted to a second measuring surface 87 viathe pressure take-off line 39. The force acting there deflects the brakevalve 81 towards a first end position 89 counter to the force of thecompression spring 88. In dependence on the resultant force of thecompression spring 88 and the oppositely directed hydraulic force, thebrake valve 81 can assume any intermediate position. As with the otherbrake valves of the exemplary embodiments of FIGS. 1 to 4, a continuousadjustment of the throttling is thus possible.

In a normal driving situation, in which, for example, the firstpump-side main line 5 a is pressurised by the hydraulic pump 2, thusonce again both the first pump-side main line 5 a is connected to thefirst motor-side main line 5 b and the second motor-side main line 6 bis connected to the second pump-side main line 6 a, the brake valve 81being deflected as far as its end position 89 on account of the forceacting on the larger second measuring surface 87, in which positionthrottling is negligible.

If, now, once again a reversal of the pressure in the main lines takesplace as a result of the pumping action of the hydraulic motor 3, thepressure acting on the second measuring surface 87 decreases and thepressure acting on a first measuring surface 90 is increased. The firstmeasuring surface 90 is, to this end, connected to the second motor-sidemain line 6 b via a connecting line 91. The hydraulic force acting onthe first measuring surface 90 acts counter to a further compressionspring 92 and displaces the brake valve 81 towards its second endposition 93.

In the second end position of the brake valve 81, likewise the firstmotor-side main line 5 b is connected to the first pump-side main line 5a and the second motor-side main line 6 b is connected to the secondpump-side main line 6 a. On account of the surface ratios of the secondmeasuring surface 87 and the first measuring surface 90, the deflectiontowards the second end position 93 on reversal of the pressure is less,so that only a throttled connection is produced between the secondmotor-side main line 6 b and the second pump-side main line 6 a, andthis connection causes the desired braking action.

To produce a braking action on reversal of the flow direction in thehydraulic circuit, a smaller measuring surface 94, having the sameorientation as the second measuring surface 87 and connected to thefirst motor-side main line 5 b via a further connecting line 95, isprovided. In order to move the brake valve 81 towards its second endposition 93 when the first pump-side main line 6 a is pressurised by thehydraulic pump 2, a third measuring surface 96, pressurised withpressure medium via a second pressure take-off line 39′ from the secondpump-side main line 6 a, is provided.

FIG. 5 a shows once again a modified brake valve 181, which can beemployed in the exemplary embodiment of FIG. 5 instead of the brakevalve 81. In the modified brake valve 181, the second connection 83 andthe fourth connection 85 are connected to one another in a throttledmanner in the rest position of the brake valve 181. The throttledconnection here brings about an improvement of the control stability.

The invention also embraces possible combinations of the hydrauliccircuit diagrams illustrated in the individual exemplary embodiments inFIGS. 1 to 5. In particular, for all designs of the brake valve unit, itis conceivable for the respectively smaller measuring surfaces to beacted upon via a pilot control valve. The pressure medium flow returnedto the tank volume 12 is preferably directed via a cooler (notillustrated), which ensures that the pressure medium does not heat up toa critical temperature even at great braking power. The drive(illustrated in the figures) of the hydraulic pump 2 is effected bymeans of a drive motor (not illustrated) via a drive shaft 2′. Adownstream-connected mechanical transmission, for example, of a vehicleto be driven may be connected to a drive shaft 3′ of the hydraulic motor3.

The exemplary embodiment illustrated in FIG. 6 is based on the exemplaryembodiment of FIG. 2, already explained in detail. In order to achieveincreased flexibility with regard to the braking power to be set, theinlet 49 of the pilot control valve 45 is now not directly connected tothe shuttle valve 50. Instead, a brake pressure control valve 120 isarranged between the shuttle valve 50 and the inlet 49 of the pilotcontrol valve 45.

The brake pressure control valve 120 is a 3/2-way valve. A hydraulicforce acts on a measuring surface 121 of the brake pressure controlvalve 120 towards its first end position. The measuring surface 121 ofthe brake pressure control valve 120 is connected to an outlet 123 ofthe shuttle valve 50 via a measuring line 122. Furthermore, a firstinlet 124 of the brake pressure control valve 120 is likewise connectedto the outlet 123 of the shuttle valve 50. By contrast, a second inlet125 is connected to the tank volume 12.

The brake pressure control valve 120 additionally has an outlet 126,which is connected to the inlet 49 of the pilot control valve 45. Independence on the forces acting on the brake pressure control valve 120,a control position of the brake pressure control valve 120 occurs whenthe forces are in equilibrium. The brake pressure control valve 120 herecan assume any positions between the first end position, in which thefirst inlet 124 is connected to the outlet 126, and the second endposition, in which the second inlet 125 is connected to the outlet 126.While a hydraulic force proportional to the higher pressure prevailingin the first motor-side main line 5 b or in the second motor-side mainline 6 b always acts on the measuring surface 121 of the brake pressurecontrol valve 120, the force acting oppositely on the brake pressurecontrol valve 120 may be adjusted. Thus, the pressure acting on theinlet 49 of the pilot control valve 45 is continuously adjustablebetween the pressure of the tank volume 12 and the higher of thepressures prevailing in the motor-side main lines 5 b and 6 b.

The brake pressure control valve 120 is subjected to the hydraulic forceat the measuring surface 121 of the brake pressure control valve 120such that the outlet 126 is increasingly connected to the first inlet124. In the simplest case, the force of an adjusting spring 127 acts inthe opposite direction. If, by contrast, the pressure acting on theinlet 49 of the pilot control valve 45 is to be adjustable in aparticularly flexible manner during operation, the force directedopposite the hydraulic force at the measuring surface 121 of the brakepressure control valve 120 is produced either hydraulically at a secondmeasuring surface 128 of the brake pressure control valve 120 orelectrically, for example by means of a proportional magnet 131.

As a result of the brake pressure control valve 120, the pressurepresent at the inlet 49 of the pilot control valve 45 is reduced ascompared with a direct connection of the inlet 49 to the shuttle valve50. Through this reduction of the pressure, the force acting on thefirst measuring surface 35 or the fourth measuring surface 35′ of thebrake valve 29 is also reduced. A reduction of the force acting on themeasuring surfaces 35 and 35′ results in an increase of the brakingpower, since, as already explained in the exemplary embodiment in FIG. 1and FIG. 2, with increasing pressure on the first measuring surface 35or the fourth measuring surface 35′ of the brake valve 29, thethrottling action of the brake valve 29 is reduced, and vice versa.

Via the brake pressure control valve 120, the braking power can thus beincreased by supplying, either to the second measuring surface 128 orthe proportional magnet 131, via a corresponding signal line 130, eithera corresponding control pressure or else an electrical control signal.This control pressure or the control signal may, for example, bedependent on the actuation of a brake pedal (not illustrated) or else onan inclination detected by an inclination sensor. When using aninclination sensor, the brake pressure control valve 120 is preferablyadjusted via a proportional magnet 131 with the aid of an electricalsignal produced by the inclination sensor.

A further application possibility arises when using a 2-speedtransmission. In this case, for example during the changing process, thecontrol pressure used to change a transmission step may be employed toproduce an adapted brake pressure by means of the brake pressure controlvalve 120.

The advantage of the exemplary embodiment according to FIG. 6 lies inthe possibility of continuously varying the braking power and thustaking account of the particular driving situation.

1. A control system for a hydrostatic transmission in an open circuitcomprising a hydraulic pump, provided for delivery to a first pump-sidemain line or a second pump-side main line, and a hydraulic motor,connected to a first motor-side main line and second motor-side mainline, and comprising a brake valve unit, which the first pump-side mainline is connectable to the first motor-side main line and the secondpump-side main line is connectable to the second motor-side main line,wherein the first motor-side main line or second motor-side main line,situated downstream of the hydraulic motor, is connectable to a tankvolume in a throttled manner by means of the brake valve unit independence on the pressure prevailing in said lines.
 2. The controlsystem according to claim 1, wherein the brake valve unit comprises abrake valve with a first measuring surface, and the brake valve issubjected to a brake pressure at the first measuring surface counter toa spring force, which pressure is dependent on the pressure prevailingin the first motor-side main line or second motor-side main line,situated downstream of the hydraulic motor.
 3. The control systemaccording to claim 2, wherein a pilot control valve, connected on theoutlet side to the first measuring surface of the brake valve, isprovided to produce the brake pressure.
 4. The control system accordingto claim 3, wherein the pilot control valve is connected on the inletside, via a shuttle valve, to the first motor-side main line or secondmotor-side main line, respectively.
 5. The control system according toclaim 3, wherein the pilot control valve for controlling the brakepressure is subjected to the pressure prevailing in the first motor-sidemain line or second motor-side main line, situated downstream of thehydraulic motor.
 6. The control system according to claim 2, wherein thebrake valve has a second measuring surface, which acts on the brakevalve in the same direction as the first measuring surface and which issubjected to a hydrostatic force from the first pump-side main line orsecond pump-side main line, situated upstream of the hydraulic motor. 7.The control system according to claim 1, wherein the hydraulic pump canbe connected to the first pump-side main line or the second pump-sidemain line via a traveling direction valve.
 8. The control systemaccording to claim 7, wherein for operation of the hydrostatictransmission with changing flow direction, the brake valve unit issymmetrically constructed.
 9. The control system according to claim 1,wherein the brake valve unit comprises a first brake valve and a secondbrake valve, the first pump-side main line being connectable in athrottled manner to the first motor-side main line by means of the firstbrake valve and the second pump-side main line being connectable in athrottled manner to the second motor-side main line by means of thesecond brake valve, in dependence on the pressure prevailing in thefirst motor-side main line and second motor-side main line, situateddownstream of the hydraulic motor, respectively.
 10. The control systemaccording to claim 1, wherein the first pump-side main line and thefirst motor-side main line and/or the second pump-side main line and thesecond motor-side main line are connected to one another each by a checkvalve which opens towards the hydraulic motor.
 11. The control systemaccording to claim 1, wherein the first pump-side main line and thefirst motor-side main line, and the second pumpt-side main line and thesecond motor-side main line, respectively, are connectable to oneanother in parallel via the brake valve.
 12. The control systemaccording to claim 1, wherein in a rest position of the brake valveunit, the flow path from the first motor-side main line towards thefirst pump-side main line and from the second motor-side main linetowards the second pump-side main line respectively, is interrupted. 13.The control system according to claim 1, wherein in a rest position ofthe brake valve unit, the first motor-side main line is connected in athrottled manner to the second motor-side man line.
 14. The controlsystem according to claim 7, wherein the connection to the tank volumetakes place via the traveling direction valve.
 15. The control systemaccording to claim 14, wherein the traveling direction valve has a restposition in which the first pump-side main line and the second pump-sidemain line are connected to the tank volume.
 16. The control systemaccording to claim 3, wherein the pressure present at the pilot controlvalve on the inlet side is controllable via a brake pressure controlvalve.